Firearms & Tool Mark

The Firearms Unit examines and compares bullets, cartridge cases, and shotgun shells to determine if they were fired from a particular firearm. This work is part of the forensic discipline known as Firearms Identification.

Firearms Identification is not ballistics, which is the study of projectiles such as bullets in motion. Some aspects are similar but ballistics is not the firearm examiner’s main focus. No college courses teach proficiency in firearms identification; experts learn through on-the-job training and a curriculum administered by senior firearm examiners.

What Firearms Examiners Look For
To match a bullet to a particular firearm the examiner looks for two criteria: class characteristics and individual characteristics.

Class characteristics are measurable features of a specimen that indicate a restricted group source. On bullets, the class characteristics are the rifling specifications of the barrel from which the bullet was fired. These include caliber, number of lands and grooves, direction of twist of the lands and grooves, and widths of the lands and grooves. If an evidence bullet and test bullets fired from a suspect firearm have the same class characteristics, the firearm examiner can conclude that the evidence bullet could have been fired from the suspect firearm.

Individual characteristics are marks unique to that particular firearm barrel. In a barrel, the individual characteristics are produced by the random imperfections and irregularities of the tool or tools used to produce the lands and grooves, and by use, corrosion, or damage. If an evidence bullet has the same class characteristics and matching individual characteristics as test bullets fired from a suspect firearm, the firearm examiner can conclude that the bullet was fired from the suspect firearm.

A firearm examiner must also be skilled in firearm function. An examiner needs to understand firearm types, as well as the actions employed by firearm manufacturers in producing these firearms. A firearm examiner must be able to describe to a jury how a particular firearm functions and why a particular firearm may have malfunctioned. This skill is learned through extensive training in the section, by touring firearm manufacturing facilities, and by attending armorer courses sponsored by various firearm manufacturers.

Sharing Ballistics Information
A key tool being used by the section’s firearm examiners is the Integrated Ballistics Identification System (IBIS). IBIS is a computer-based system for comparing digital images of cartridge cases found at crime scenes against digital images of fired casings found at other crime scenes and test fired casings from weapons. IBIS is linked to a national system called the National Integrated Ballistics Information Network (NIBIN). This allows the firearms examiners to compare images entered to images in any IBIS system across the country.

This can provide quick and valuable investigative information to the many law enforcement agencies in the state. The firearm examiner still bears the responsibility of making the final determination on identifications, but IBIS can link evidence in a new way.